tank vs. tankless

My hot water is provided by a ground source system, and works well. However, I'm adding an apartment over the garage, which is at least 80' away and the hot takes a full minute to arrive. And when pulling the permit, the person doing the Title 24 (California's energy conservation code) could not get the current system to calc out and I was told I must make the domestic for the apartment with gas (propane as I'm rural).

Now that I've stopped whining and am cutting open drywall to run a 3/4" gas line, I don't know whether to go tank or tankless. The forum has lots of grumbling over tankless and I've had mixed success in the past myself. However the best location for the heater is an attic over the stairwell and is only 4' high. Although the vent can exit straight up through the roof there, the inspector told me he didn't think any gas tank heater would legally fit in that short a space. If true, a demand heater it is.

In conjunction with this, should I use the geothermally heated water as the supply for whatever heater I end up installing or use the cold for the supply?

To me, it would somewhat depend on what fixtures were there in the apartment. If it is just a shower and say a kitchen, and only a shower, not a tub, a tankless could work fine. Hassle is the required maintenance (periodic deliming). Depending on your water coming in, that could be fairly frequent. If you have a softener, probably not very often.

What are you doing for heat in the apartment?

If it's just that it takes too long to get hot water there, why not just put in a recirculation system? To save some, insulate the lines, and run it on a timer.

Also you have to take in consideration the amount of LP gas you will be consuming to heat the water. Where a standard atmospheric vent 40 Gal heater would only burn around 40,000 BTU give or take a few depending on the brand and model. Where a Tankless unit worth its weight will be burning 200,000 BTU's

Granted the Tankless only runs when the water is running, and shuts off right after the water is turned off. Where a tank heater will run till it gets all 40 gallons up to temperature. Here is the problem I heard a few of my customers complain about. They stopped seeing a savings in the gas bill due to the fact they have an endless supply of hot water with their Tankless unit and take much much longer showers. And as jadnashua said you have to delime the Tankless on a regaler basis.

To me, it would somewhat depend on what fixtures were there in the apartment. If it is just a shower and say a kitchen, and only a shower, not a tub, a tankless could work fine. Hassle is the required maintenance (periodic deliming). Depending on your water coming in, that could be fairly frequent. If you have a softener, probably not very often.

What are you doing for heat in the apartment?

If it's just that it takes too long to get hot water there, why not just put in a recirculation system? To save some, insulate the lines, and run it on a timer.

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This issue is that it won't meet CA Title 24 efficiency standards to be a legal install, and with a recirculator it probably still wouldn't, even with insulated pipes. (Unless you do a GREAT job and produce an engineering study to back it up how well you've mitigated the losses.)

Whether or not it's worth the trouble to feed the tankless with geothermal pre-heat depends on the anticipated prices for electricity and propane over the next decade, as well as the installation price. But unless the water line is insulated very well (2" of closed-cell foam) you'll be dumping a lot of hard-won geothermal heat back into the ground.

For the money (and given the amount subsidy), some amount of solar (even batch) pre-heating ahead of the tankless might make sense, if you think propane pricing over the years will be more like 2007-2008 than 2009 so far.

FWIW: Periodic deliming may/may-not be necessary. It really depends on the particulars of the water system & sources. Hard water areas definitely need occasional deliming, but in other areas (including my own place) they can go 15+ years without liming/scaling issues. If you already need/have a water softening system and KNOW it'll be an issue, plumbing in valves & ports for the purpose when it's installed can take the pain out of deliming rinses.

A propane-fired tankless will always outperform a propane fired tank. How much depends on the absolute use volume. Tank heater combustion efficiency tops out at ~80%, and even the lowest efficiency tankless burners are in the low-mid 80s, so best-case (as in space-heating application) a tank HW heater delivers high-70s performance to a tankless' low 80s, not huge. It's the extremely high standby losses that kill the tank efficiency in domestic hot-water applications.

A tankless has no continuous standby losses, and low single-digit percentage losses cooling between intermittent daily draws. The bigger losses with a tankless occur during flue purges at the beginning/end of firing, making very short draws fairly lossy on a percentage-of-fuel burned vs. heat delivered basis. A 5 gallons or bigger draws they all pretty much hit their EF test numbers, which is within a couple percent of their steady-state thermal efficiency. (A tank's burner has to run 23 out of 24 hours to hit that close to it's steady state efficiency, and even that is a 4-5% lower efficiency than a lowest-efficiency tankless.)

"Never ending shower" issues notwithstanding, most users see double-digit percentage drops in fuel use going from a tank to a tankless. Low volume users (like single guys who never bathe ) can see 50% drops in fuel use. Even 120gallons+/day families will see mid-single-digit savings. Clearly YMMV

If you go the tankless route, price out the difference between the fully-installed price of a condensing vs. non-condensing (90%+ ) version. Often it's not very different, due to the fact that the condensing unit can use cheap PVC flue venting, whereas the lower-efficiency version requires stainless steel flues, which can add $1000 to the up-front hardware costs. If the vent has to run a significant distance, the condensing unit's installed price can even be lower. A condensing tankless will typically beat a non-condensing tankless by ~10% in real-world performance, beating a tank by 25%+ every time. Even if that's only $75/year savings vs. a tank, it sure beats only $30/year savings with a bottom-efficiency tankless in the same installation (and is usually worth paying a couple hundred more for, if that's where the numbers fall.)